The present invention is directed to machine vision and in particular to providing illumination for such systems.
Machine vision has been applied to a number of production and testing tasks. In general, workpieces, such as printed-circuit boards, integrated-circuit chips, and other articles of manufacture are brought into the field of view of a camera. The camera typically generates an image in digital form, which digital circuitry normally in the form of a microprocessor and related circuitry processes in accordance with the task to be performed.
In many cases, the workpiece is too large for a practical-sized camera to image with adequate resolution, but this problem is readily solved by taking an image of only a small part of the workpiece at any single time. This yields the requisite resolution, and is images of respective segments of the workpiece can be taken as the workpiece is stepped through the camera""s field of view.
Although this approach is acceptable in a number of applications, it can be throughput- and accuracy-limiting in some others. There are often practical limits to the speed at which the workpiece can be advanced through the camera""s field of view. Additionally, the need for accurate correlation between successive images can impose severe accuracy requirements on the workpiece-advancing system. To a greater or lesser degree, the same limitations apply regardless of whether it is the camera or the workpiece that is moved.
For some applications, a superior solution is to move neither the camera nor the workpiece, but rather to move the camera""s field of view by employing deflector mechanisms. Galvanometer-mounted pivoting mirrors, pivoting prisms, and rotating reflector polygons are among the mechanisms commonly employed in optical systems to perform image deflection. Although these still are moving parts, they are ordinarily relatively small and take advantage of optical leverage to change the field of view faster than systems that move the entire workpiece or camera.
Despite this advantage, there is a class of applications to which workers in this field have been slow to apply the field-of-view-deflection approach. One example of this class is the type of application that involves reading laser-scribed marks on workpieces such as semiconductor wafers or electronic-component packages. Marks of that type are hard to detect reliably because they are quite subtle. So considerable effort has been applied to illuminating the workpiece in such a manner as to minimize noise contributed by surface irregularities in non-marked regions. But achieving this result is greatly complicated in systems that use field-of-view deflectors. In systems that move the workpiece or the camera, the illumination apparatus always has the same position with respect to the field of view, so illumination characteristics need to be optimized for that relationship only. In field-of-view-deflector systems, on the other hand, the lighting system would have to be optimized for a wide range of resultant relationships between the lighting system""s position and that of the camera""s field of view. For some applications, the difficulty of solving this problem has confounded attempts to employ field-of-view deflection.
But we have recognized that imaging results for such systems can be greatly improved by emphasizing the dark-field-illumination aspects of the problem and adapting to it a method previously used to vary dark-field illumination in response to camera-objective changes.
xe2x80x9cDark-field illuminationxe2x80x9d is an illumination approach that takes advantage of the fact that a specularly reflecting feature in the midst of a diffuse-reflecting background will appear dark if that feature""s specular reflection images the main light source outside the camera""s field of view. That is, since the angle of reflection of all light striking a specular reflector equals that light""s angle of incidence, the reflected light will not pass through the camera""s entrance pupil unless that sole angle of reflection yields that result. But light striking the diffusely reflecting background is reflected in a range of angles, so a substantial amount may enter the camera even if the specular-reflection angle would not result in a ray that does. The specularly reflecting feature is therefore readily identified because it appears dark against a lighter background.
Because of this effect, there is a rich store of work directed to dark-field illumination, and we have recognized that properly adapting it can yield significantly improved results for field-of-view-deflection systems. It had long ago been recognized in systems such as that described in U.S. Pat. No. 4,604,648 to Kley that individual elements of a light-source array should be selectively operated in accordance with the particular objective or zoom position of the imaging camera. We have adapted this concept by so operating elements of a light-source array that the position within the array at which one or more light sources is not lit moves around the array as the deflector changes the field of view""s position.
Specifically, as the deflector so moves as to change the field of view on the workpiece, we selectively turn off any elements of the source array that will be imaged into the camera""s field if specular reflection occurs in the portion of the workpiece within that field of view. As the field of view moves so that an element previously thus imaged no longer is, the element is typically turned on again.